The transmission capacity of fiber-optic communication systems has been increased significantly by the use of wavelength division multiplexing (WDM) techniques. In a WDM communication system, multiple channels—where each channel is differentiated by using a unique wavelength of light—carry modulated optical signals in a single optical fiber between a transmitter and a receiver. The transmitter uses an optical multiplexer to combine multiple channels into the fiber for transmission, and the receiver uses an optical demultiplexer to separate the optical channels for detection. A typical optical demultiplexer (demux) contains a single input port and multiple output ports, where each optical channel from the input port is mapped to a unique output port in sequential order. One such optical demultiplexer is shown in U.S. patent application Ser. No. 09/944,800, which is incorporated by reference as if fully set forth herein. Optical multiplexers are simply demultiplexers operated in the reverse direction, where a specific wavelength has to be supplied to the correct input port to emerge at the output port as part of a multiplexed signal.
U.S. patent application Ser. No. 09/944,800 discloses a programmable optical multiplexer/demultiplexer having multiple ports that can establish an independent reconfigurable connection between any two of the device's ports for each optical wavelength that is supplied to the device as part of a wavelength division multiplexed signal. In one arrangement disclosed therein, a programmable demultiplexer receives an input signal containing components at N different wavelengths at a port of the arrangement that acts as an input port, and distributes the input signal components among K ports that act as output ports. The input signal is collimated by a particular microlens in a microlens array, the particular lens being aligned to the input port. The microlens array contains K additional microlenses, each of which is aligned with a respective one of the K output ports. The resultant collimated beam originating from the input port is then made incident on a diffraction grating, which angularly disperses the composite optical signal according to wavelength, thereby forming N separate beams, each being at a different wavelength and having a distinct propagation angle. Each of the N separate beams propagates to a single lens that is arranged to collect all the beams and provide, for each wavelength, a converging beam focused onto a particular micro mirror in an array containing N micro mirrors. Each mirror in the array is individually controlled to reflect the incident beam, which is at least one of the various wavelengths, in a desired direction, such that it will (a) re-enter the lens, (b) be collimated by the lens and redirected to a different location on the diffraction grating, and (c) be eventually coupled from the diffraction grating through a particular microlens in the microlens array to a desired output port, the particular microlens being aligned with the desired output port.
Generally, the number of output ports K and optical wavelength components N are independent. The demultiplexer can be designed to operate in the regime where K=N, so that each wavelength component can be assigned to any output port. The arrangment can also be operated in a mode where K<N, in which case more than one wavelength is applied to an output port, or in a mode where K>N, in which case one or more output ports are not used. In any event, the arrangement enables assignment of any wavelength to any output port Such an arrangement may also be operated in the “reverse” direction, in order to act as a programmable multiplexer, rather than as a demultiplexer. In the multiplexer arrangement, K input signals, each containing one or more different wavelengths, are received from a plurality of K optical input ports and must be combined and made available at a single output port. The K input signals cumulatively contain a total of N different wavelengths, or, stated differently, any particular wavelength component can exist at only one of the K input ports. If this criterion is not met, contention will occur. Each input signal is collimated by a respective microlens in a microlens array that contains K+1 microlenses. One microlens is aligned with the output port, while the remaining microlenses are aligned each to a corresponding input port. The resultant collimated beam originating from each input port is then made incident on a diffraction grating, which diffracts the optical signal as a function of its wavelength. The diffraction grating is arranged such that all of the separate beams, which have different wavelengths and therefore distinct propagation angles, propagate to a single microlens that collects all of the beams and provides, for each wavelength, a converging beam focused onto a particular micro mirror in a micro mirror array. Each micro mirror in the array is individually controlled to reflect the incident beam, representing a corresponding wavelength in the desired direction, such that it will (a) re-enter the microlens, (b) be collimated by the microlens and redirected to a single location on the diffraction grating, and (c) be eventually coupled from the diffraction grating to the output port through the particular microlens in the microlens array that is aligned with the output port.
Again, in general, the number of input ports K and optical wavelength components N are independent. The multiplexer can be designed to operate where K=N, so that each wavelength component can originate at any input port. The arrangment can also be operated in a mode where K<N, in which case more than one wavelength is applied to an input port, or in a mode where K>N, in which case one or more input ports are not used. In any event, the arrangement enables multiplexing of all input wavelengths originating at the K input ports to the output port.
Disadvantageously, such an arrangement is too expensive for a small number of wavelengths, because it requires the same costly precise alignment of all components independent of the number of wavelengths employed, so the per-wavelength cost is high for a small number of wavelengths. Such an arrangement is also relatively inflexible in regards to its wavelength splitting abilities. More specifically, the bandwidth is distributed homogenously over a plurality of micro mirrors. The mirror dimensions must be chosen to correspond to the desired wavelength bandwidth. This makes it desirable to have the smallest possible spacing between the mirrors. Furthermore, if there is a gap between the mirrors, there will be a gap between the wavelengths.